Our U.S. Pat. No. 4,451,366 discloses a separator which is particularly suitable for separating, for example, sewage and other solid matter from water in stormwater overflows. The separator is in the form of a cylindrical vessel having an inlet which is disposed tangentially so as to promote a circulating flow within the vessel. This circulating flow comprises an outer, relatively fast flow and an inner, relatively slow flow, the shear zone between these two regions being stabilized by an annular dip plate which projects downwardly from the top of the vessel. A flow-modifying member is provided in the vessel to enhance the removal of solid particles accumulating at the bottom of the vessel to a central outlet. Clean water is removed from the top of the vessel.
Because the separator disclosed in U.S. Pat. No. 4,451,366 was originally designated for use in stormwater overflows, two principal requirements were that it should operate at low energy levels (i.e. with a low pressure head at the inlet) and that it should be maintenance free. The latter requirement means that the use of moving parts, and in particular of active energy sources such as pumps, has hitherto been avoided. Consequently the energy input to the known separator has been derived entirely from the kinetic energy of the flow into the vessel.
Two somewhat distinct processes take place in the known separator. Firstly, solid matter in the incoming mixture is allowed to fall out of the liquid (usually water), so achieving separation of the solid matter from the liquid. Contrary to what might initially be thought, centrifugal forces resulting from the circulating flow of the mixture in the vessel play an insignificant part in this separation process. Separation occurs almost entirely under the force of gravity acting on the particles, and to achieve a high separation efficiency the vessel is designed so that each "packet" of mixture entering the vessel travels along the longest possible path before reaching one of the outlets, so allowing enough time for the solid particles to fall to the base.
The second process which takes place in the separator is the removal from the vessel of the solid matter which reaches the base. This removal process is achieved under the action of boundary layer effects at the base of the vessel; solids reaching the base are entrained in the laminar sub-layer at the base and progess, usually in the form of migrating dunes, towards the center of the vessel. In the separator of U.S. Pat. No. 4,451,366, the flow-modifying member assists this migration by creating an inwards sweeping effect through an annular slot defined between the flow modifying member and the base of the vessel.
In the design of the known separator, there is an inherent conflict in the dual requirement for efficient separation and efficient solids removal. Separation efficiency is improved by positioning the inlet at a relatively high level, and preferably above the lower edge of the dip plate. Separation efficiency is also improved by extending the dip plate downwardly for a considerable distance, for example for 75% of the total height of the vessel. However, extending the dip plate in this way gives rise to a considerable energy loss owing to friction effects at the surface of the dip plate. Furthermore, when running at optimum separation efficiencies, the energy available for solids removal decreases to very low values, and blockage of the solids outlet can become a problem. Experience with the known separator has shown that there is a very rapid drop in velocity between the inlet and the base. It will be appreciated that energy losses will be greater as the viscosity of the fluid in the vessel increases. For efficient solids removal, the intake should be positioned low down the vessel to transfer as much energy into the boundary layers at the base as is possible. The dip plate should be as short as possible to reduce friction energy loss. It is not possible to meet these conflicting requirements with the separator disclosed in U.S. Pat. No. 4,451,366 and that separator is consequently a compromise with the intake disposed substantially halfway down the vessel, and with a relatively short dip plate.
One way of increasing the energy input and operating head is to throttle the inlet. However, at very low flow rates, sufficient energy to establish the correct flow pattern in the vessel can only be achieved by reducing the area of the inlet to such an extent that particles in the inlet flow can block the inlet. Furthermore, it is not entirely certain that the achievement of the correct flow pattern in the vessel is dependent solely on the kinetic energy input. It is believed that angular momentum may also be a significant influence.